Applied In process measurements technology and in the field of gas- and liquid analysis are measuring systems for registering physical and/or chemical to measured variables. Measuring transducers are components of such measuring systems. Important measured variables in process measurements technology and in gas-, respectively liquid, analysis are temperature, pressure, flow and fill level, as well as especially analytical parameters of measured media, e.g. their pH-value, conductivity, concentrations of certain ions or other chemical substances, such as, for example, oxygen, carbon dioxide, organic substances or nutrients in the measured media. These analytical parameters play a role in various applications, for example, in laboratory or in process, respectively analytical measurements, technology in the fields of chemistry, pharmacy, biotechnology, food technology or environmental technology.
Fundamentally, a measuring transducer transduces the registered measured variable into an electrical signal, which is correlated with the measured variable via a characteristic curve for the measuring transducer. The measurement signal present firstly as an electrical signal, for example, a measurement voltage, can be further processed by means of an evaluation circuit and output in the units of the measured variable to be ascertained and displayed.
Measuring systems used in process measurements technology or in analytical measurements technology can comprise a housing, in which are integrated the measuring transducer, the evaluation circuit and a display device. For more complex evaluations, especially for storing and/or processing measured values and/or for control of processes using the measured values registered by the measuring system, the measuring system can include means for data processing. This can be embodied, for example, as a data processing system in the form of a measurement transmitter, a computer or a programmable logic controller. Applied in analytical measurements technology in many applications are measuring transducers, whose lifetime is significantly shorter than that of the display apparatus or the means for data processing. This is true, for example, in the case of measuring transducers such as pH-sensors, ion-selective electrodes, optical or amperometric sensors for registering concentrations of certain substances in the measured medium. Frequently in these applications, the measuring transducers are embodied as exchangeable units, e.g. in the form of measuring probes, which are separate from the display device or the means for more extensive data processing, and communicate with these via a cable connection or wirelessly. In such case, at least a part of the evaluation circuit, for example, the on-site electronics, can be a component of the exchangeable measuring transducer unit.
Real measuring transducers deviate with time ever more strongly from ideal behavior due to aging from the influence of external conditions loading the measuring transducer, as well as also due to inner changes. This deviation from ideal behavior results in a shifting of the measuring transducer characteristic curve. It is, consequently, established practice to service the measuring transducer from time to time and, in given cases, to perform a compensation of the deviation. This is quite usual in the case of electrochemical measuring transducers, such as pH-electrodes, ion-selective electrodes, amperometric oxygen sensors, especially dissolved oxygen sensors, as well as other amperometric measuring transducers and also in the case of conductivity sensors. Such a compensation, in the case of which the display value of the measuring transducer is adjusted to the true value of the measured variable, is referred to as adjustment. Since in process measurements technology, however, the not quite fitting terminology “calibration” is, as a rule, used for this procedure, this terminology will be maintained here and in the following.
The end of the lifetime of the measuring transducer is reached when its aging has progressed so far that, in spite of calibration, reliability of the measured values delivered by the measuring transducer is no longer assured. In this case, depending on type of measuring system, either the entire measuring system is taken out of operation and replaced by a new one, or the measuring transducer is replaced.
The aging of a measuring transducer, which leads to a change of the transfer function, depends also on environmental conditions, to which the measuring transducer is exposed. Thus, it is known, for example, that high temperatures accelerate shift the aging process. Also, measuring transducers, which during operation come in contact with chemically aggressive, measured media, for example, strong acids or alkaline solutions, or which are exposed to strong mechanical loadings, e.g. measuring transducers, which are exposed during operation to a medium with high entrained dirt loads or high pressures, can disproportionately rapidly age.
Described in German patent, DE 101 41 408 A1 is a method for determining the calibration interval time, i.e. amount of time between two calibrations, of electrochemical measuring transducers. The method explained in greater detail using the example of a pH-sensor includes sequential registering during the operation of the measuring transducer of at least one measurement parameter relevant for judging the aging of the sensor. Examples of such relevant measurement parameters include temperature and pH-value. A predetermined base calibration interval time, which is fixed under the proviso that the monitored measurement parameters lie in a basic value range only little influencing the aging of the sensor, can be adapted based on the registered measurement parameter in an ongoing manner when extreme values of the measurement parameter occur, which lead to an accelerated aging of the measuring transducer.
This method takes into consideration, indeed, the influence of extreme values of the monitored measurement parameter on the aging of the measuring transducer, however, it neglects the disproportionate influence of especially demanding processes, since it is not designed to identify such processes and correspondingly to take them into consideration.
Described in European patent, EP 1 550 861 B1 is a method for determining the state of a measuring transducer, which is integrated in a containment, and which is cleaned from time to time without being deinstalled, for example, using known CIP- (cleaning in place) or SIP- (sterilization in place) methods. The method includes monitoring the temperature as a function of time, wherein based on the temperature as a function of time, especially in comparison with threshold values, it is detected that a CIP- or SIP method has taken place. The loading of the measuring transducer associated with the established method is registered, the sum all loadings ascertained and by comparison with a maximum value of the allowable loadings, an allowable remaining loading or remaining life is calculated.
This method enables consequently the identification of CIP- or SIP-processes, to which the measuring transducer is exposed, and taking such into is consideration in determining the state of the measuring transducer.
Disadvantageous, however, is that the state of the measuring transducer is determined based on temperature, respectively the process acting on the measuring transducer, based on the load resulting therefrom for the measuring transducer. The determining of sensor state based on the loadings experienced by the sensor can, however, only be a coarse estimation, which neglects that the aging of various examples of the same sensor type is influenced in different degrees by one and the same loading, due to manufacturing related variations. Since the method described by EP 1 550 861 B1 does not take such sample variations into consideration, considerable error can occur in determining the state of the measuring transducer.